The present invention relates to digital data filtering, and more particularly to a simple coefficient half-bandwidth digital filter which provides digital data rate compression.
In the quest for data rate reduction of a digital signal for transmission, the lowest possible sampling frequency should be employed prior to any other algorithm. This rate was defined by H. Nyquist of Bell Laboratories as twice the highest expected frequency. The problems associated with sampling at the Nyquist rate, i.e., 8.5-9.0 MHz for video data where the highest expected frequency is 4.2 MHz, are well understood. Anti-aliasing filtering of the input signal is difficult for linear phase and step attenuation characteristics to be achieved. As a practical matter the Nyquist sampling rate is extremely difficult to achieve. Prior analog anti-aliasing filters have inherent well-known problems and have a lowest practical sampling rate of four times the video subcarrier frequency, with sampling rates of as low as three times video subcarrier frequency being accomplishable at the expense of extremely complex circuitry. Likewise digital filtering has been perceived as being too complex. Digital finite impulse response (FIR) filters process a signal along a multi-element delay line, multiplying the output of each element by a weighting factor and summing all the weighted outputs together to produce an output signal.
What is desired is a digital data rate compression system which is simple and provides a lower sample rate than prior systems.